Stop displaying layers with non-invertible transforms.

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index c1bc1a12c4dff940fccb6e5f4c4bbb2e89d79213..d16d5a734258ec661823edcaf127799c35102d3c 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -478,6 +478,10 @@ static bool LayerShouldBeSkipped(LayerType* layer, bool layer_is_drawn) {
 
 static inline bool SubtreeShouldBeSkipped(LayerImpl* layer,
                                           bool layer_is_drawn) {
+  // If the layer transform is not invertible, it should not be drawn.
+  if (!layer->transform_is_invertible())

[ajuma, 2014/03/28 18:49:07]

Can this early-out interact negatively with readback requests (see the next
check below)? Should we also check if transform is animating here (just as we do
below before checking for opacity 0)?

[avallee, 2014/04/15 18:53:25]

Yes, we do. The spec is all about requiring that it's not rendered. It shouldn't
show up in readback or at any singular in-between transform.

[ajuma, 2014/04/15 19:45:10]

I agree with not drawing it, but I'm worried that if we just skip the layer here
altogether even when we're animating it, this will make it impossible to solve
issues like crbug.com/354880, where we want to pre-rasterize layers that are
animating scale from 0 (since we'll end up skipping the subtree that we want to
pre-rasterize). Can we prevent drawing such an animated layer without skipping
it here?

[avallee, 2014/04/16 18:20:56]

Done. Won't skip on animated transform. Fix for second CL.

+    return true;
+
   // When we need to do a readback/copy of a layer's output, we can not skip
   // it or any of its ancestors.
   if (layer->draw_properties().layer_or_descendant_has_copy_request)
@@ -501,6 +505,10 @@ static inline bool SubtreeShouldBeSkipped(LayerImpl* layer,
 }
 
 static inline bool SubtreeShouldBeSkipped(Layer* layer, bool layer_is_drawn) {
+  // If the layer transform is not invertible, it should not be drawn.
+  if (!layer->transform_is_invertible())

[ajuma, 2014/03/28 18:49:07]

Same questions as above about readback and animating transforms.

[avallee, 2014/04/15 18:53:25]

See above.

+    return true;
+
   // When we need to do a readback/copy of a layer's output, we can not skip
   // it or any of its ancestors.
   if (layer->draw_properties().layer_or_descendant_has_copy_request)
@@ -1070,6 +1078,12 @@ static void PreCalculateMetaInformation(
   bool has_delegated_content = layer->HasDelegatedContent();
   int num_descendants_that_draw_content = 0;
 
+  if (!layer->transform_is_invertible()) {

[ajuma, 2014/03/28 18:49:07]

Should we check if transform is animating? (e.g. Suppose we're animating scale
from 0 to 1. Do we really want to skip this layer when this is run on the main
thread layer tree?)

[avallee, 2014/04/15 18:53:25]

Yes we do.

[ajuma, 2014/04/15 19:45:10]

We do want to skip, or we do check? (I don't see this checked in the latest
patch.)

[avallee, 2014/04/16 18:20:56]

Let's check for now and this will need to be changed for pre-rastering.

+    // Layers with singular transforms should not be drawn, the whole subtree
+    // can be skipped.
+    return;
+  }
+
   if (has_delegated_content) {
     // Layers with delegated content need to be treated as if they have as
     // many children as the number of layers they own delegated quads for.